Intermolecular forces are the attractions between separate molecules, and they're what actually determine physical properties like boiling point, not the covalent bonds holding each individual molecule together.
London dispersion forces
Present in every single molecule, polar or nonpolar, dispersion forces come from temporary, constantly shifting uneven electron distributions that create brief instantaneous dipoles. Their strength increases with molar mass and surface area, since more electrons and more contact area mean a stronger temporary attraction.
Dipole-dipole forces
Between polar molecules, the permanent partial positive end of one molecule attracts the permanent partial negative end of another. This only applies to molecules with a genuine net dipole, not just polar bonds that cancel out symmetrically.
Hydrogen bonding
A particularly strong version of dipole-dipole attraction, hydrogen bonding requires hydrogen bonded directly to nitrogen, oxygen, or fluorine, the three most electronegative elements small enough to create an especially strong, concentrated partial charge.
Ranking overall strength
As a general rule, hydrogen bonding is strongest, then dipole-dipole, then dispersion. But this ranking isn't absolute: a large enough nonpolar molecule can have dispersion forces that outweigh the dipole-dipole forces in a much smaller polar molecule, which is exactly the kind of comparison AP Chemistry likes to test.
Where students actually lose points here
Confusing intermolecular with intramolecular forces
Intramolecular forces are the actual bonds inside a molecule, ionic or covalent. Intermolecular forces are the weaker attractions between separate molecules. A question about boiling point wants intermolecular forces, not bond strength.
Forgetting dispersion forces exist in polar molecules too
Every molecule has dispersion forces. Polar molecules just have dipole-dipole forces layered on top of them, not instead of them.
Calling any dipole-dipole attraction "hydrogen bonding"
Hydrogen bonding specifically requires hydrogen attached to nitrogen, oxygen, or fluorine. A dipole-dipole attraction involving any other atoms is simply dipole-dipole, not hydrogen bonding.
The explanation behind physical properties
Boiling point, melting point, vapor pressure, and viscosity all trace back to intermolecular forces, which makes this topic quietly foundational for the rest of Unit 3.
Common questions
What's required for hydrogen bonding to occur?
Hydrogen must be bonded directly to nitrogen, oxygen, or fluorine. Those three elements are electronegative and small enough to create an especially strong, concentrated partial charge.
Can dispersion forces be stronger than dipole-dipole forces?
Yes. A large nonpolar molecule with many electrons can have stronger dispersion forces than a small polar molecule's dipole-dipole forces, since dispersion force strength increases significantly with molar mass and surface area.
What's the difference between intermolecular and intramolecular forces?
Intramolecular forces are the actual bonds inside a molecule, like covalent or ionic bonds. Intermolecular forces are the weaker attractions between separate molecules, and they're what control properties like boiling point.